Think of widgets as configurations for pieces (called elements) of an app's UI.
An element is a widget that’s been made real and mounted on-screen.
The element tree represents what is actually displayed on the device at any given moment. Note that there is also a parallel widget-tree.
An app has two trees, one that represents what’s actually on the screen (the elements), and one that holds the blueprints they were made from (the widgets).
A Flutter app is at its root, a StatelessWidget the main function calls the runApp on.